Semiconductor laser devices having an active layer (light emitting region) employing a multiple quantum well structure are known in the art. Such devices emit light at lower threshold currents than semiconductor laser devices having a bulk active layer, and have a higher optical output power.
An example of a known MQW laser device is shown in cross-section in FIG. 1a. The laser device depicted in FIG. 1a comprises a number of semiconductor layers which are formed using known techniques on a semiconductor substrate 1, which in the example depicted is an n-type semiconductor. The various layers include: an n-type lower cladding layer 2A, an undoped lower optical confinement layer 3A, an active layer 4, an undoped upper optical confinement layer 3B, a p-type upper cladding layer 2B, and a p-type cap layer 5. These layers are sequentially formed on the substrate 1 by any of a number of known epitaxial crystal growth techniques such as, for example, metal organic chemical vapor deposition (MOCVD). An n-type lower electrode 6A is also formed on the lower side of substrate 1 and a p-type upper electrode 6B is formed over cap layer 5. As depicted in FIG. 1a, active layer 4 and the adjacent optical confinement layers 3A and 3B are formed into an elongated mesa structure using standard photolithographic techniques. Current blocking p-type semiconductor layer 7A and n-type semiconductor layer 7B are then formed in the region adjacent to the mesa structure, so that, in operation, current is injected into active layer 4 in a narrow area.
The resulting structure is then cleaved to provide a laser having a predetermined cavity length (L), with a front facet (S1), used for light emission, formed on one cleaved plane and a rear facet (S2) formed on the opposite cleaved plane. The front facet S1 has an antireflective coating to facilitate light emission from the front surface of the cavity and the rear facet has a highly reflective coating to suppress light emission from the rear surface.
It is known that the active layer 4 may be designed to have a MQW structure consisting essentially of alternate hetero-junctions of well layers made of semiconductor material. Each hetero-junction comprises a pair of semiconductor layers: a well layer of a narrow band gap energy and a barrier layer. The barrier layer has a band gap energy which is wider than that of the well layers. Each of the various sub-layers in the MQW structure has a thickness of several nanometers.
Lower and upper optical confinement layers 3A and 3B adjacent to active layer 4 are each designed to have a separate confinement heterostructure (SCH), in order to enhance the confinement of the laser light generated in active layer 4, thereby enhancing the external differential quantum efficiency of the laser to achieve high optical output power operation.
It is also known in the art that the semiconductor laser device of FIG. 1a may be mounted in a package to form a laser module which is suitable for use as a signal light source in an optical communications system, or as a light source for pumping an optical fiber amplifier such as an erbium-doped fiber amplifier (EDFA) or a Raman amplifier. Within the package, the laser device may be thermally coupled to a cooling device comprising Peltier elements. The package may also include any other known elements to monitor and control heat generation and optical output, and to ensure good optical coupling of the laser output to an optical fiber.
In recent years, the rapid growth in the Internet and other communications systems has led to the development of fiber-optic wavelength division multiplexing (WDM) system architectures to provide increased data transmission capacity in such systems. In order to provide optical fiber amplifiers with enhanced optical output performance to meet the demand for an increased number of channels, there has been a need for pumping lasers with high optical output coupled to the optical fiber. Pumping lasers for optical fiber amplifiers are required to offer stable operation with ever higher fiber-coupled optical output and with narrower spectral width, especially for use in optical fiber Raman amplifiers.
One way to achieve a high optical output pumping laser with an MQW active layer is to increase the cavity length (L). Increasing the cavity length decreases both the electric resistance and thermal impedance of the laser device. This results in a larger saturation driving current Isat at which the maximum optical output power occurs, since the saturation is dominated by thermal saturation effects. However, for a given value of output facet reflectivity, increasing L causes the external differential quantum efficiency to lower, as depicted in FIG. 1b. As can be seen in FIG. 1b, the power versus current slope is initially lower for long cavity lasers. Thus, it can be disadvantageous to use a long cavity laser for high optical output power operation at certain driving currents, because the external differential quantum efficiency decreases as the cavity length increases.
This problem can be ameliorated to some degree by reducing the reflectivity of the output facet. However, lowering the reflectivity of the output facet of the laser device below a certain value results in a decrease of the differential quantum efficiency of the device and a decrease of maximum optical output power. Reported mechanisms for such a decrease include carrier leakage from the MQW structure to the optical confinement and cladding layers, increased optical absorption loss and recombination carrier loss at the confinement layers due to the carrier leakage and non-uniform hole injection into the MQW structure.
The graded index, separate confinement heterostructure (GRIN-SCH) is known to be effective in suppressing deterioration in low power, short cavity lasers. Continuous GRIN or multi-layer GRIN structures have been reported in order to realize low threshold current operation. For high power lasers, the reported results have shown that a two step GRIN structure has an advantage of high optical power.